1. Field of the Invention
The present invention relates generally to a method for carrying out a remelting/hardening treatment. More particularly, the present invention relates to a method for carrying out such remelting/hardening treatment for a preheated work with the aid of a torch, wherein when the work is preheated at a lower temperature, a preheating operation is additionally performed for the work by using the torch and the remelting/hardening treatment is then carried out on the properly preheated work.
2. Description of the Prior Art
It is known in metallurgical technology to chill a metallic structure of a surface of a work or article made of cast iron or cast steel by allowing the surface of the work to be subjected to remelting and hardening, thereby to improve wear resistance and shock resistance of the work.
For example, a procedure for carrying out remelting/hardening treatment across the full width of a cam surface by rotating a preheated cam shaft while reciprocatively displacing a preheating torch in the axial direction of the cam shaft has been disclosed in an official gazette of Japanese Unexamined Patent Publication (Kokai) No. 60-258421.
According to such prior procedure, when the cam shaft is not preheated within a predetermined or particular temperature range before the cam shaft is subjected to the remelting/hardening treatment, it has been reported that there arises a malfunction such as surface crack or the like due to cooling at an excessively high speed after completion of a melting operation. For this reason, it is necessary to carefully control a preheating temperature (usually ranging from 150.degree. to 400.degree. C.) of the cam shaft which is to be subjected to remelting and hardening. Generally, a preheating station is arranged on the upstream side in a remelting/hardening treatment line along which cam surfaces on the cam shaft are successively subjected to remelting and hardening, at a plurality of treatment stations arranged on the downstream side of the remelting/hardening treatment line.
With respect to the foregoing remelting/hardening treatment line in which each cam shaft is successively subjected to remelting and hardening, there is a possibility that a temperature of the cam shaft is lowered during treatment in a plurality of treatment stations even through the cam shaft is correctly preheated to a predetermined temperature in the preheating station. In addition, there is a possibility that the temperature of the cam shaft is lowered due to unexpected line stoppage or similar malfunction.
Once the temperature of the cam shaft is lowered, there arises a problem that the remelting/hardening treatment will be carried out on the cam shaft that no longer is at a sufficiently high preheating temperature, in the event that the temperature of the cam shaft is not measured at each of the treatment stations. As a result, an unacceptable cam shaft may be produced undesirably.
On the other hand, in the event that the temperature of each cam is measured in the respective treatment station. If at any such station the preheating temperature is too low, the cam shaft may be returned to the preheating station. In such case, however, a conveying system extending across the remelting/hardening treatment line becomes complicated in structure. Also, many man-hours are consumed for returning insufficiently preheated cam shafts to the preheating station. Further, the remelting/hardening treatment is carried out at a low operational efficiency. Moreover, cam shafts can not smoothly be delivered to a subsequent step, i.e, an engine assembling step.